Abstract The combination of metal ions with proteins offers unique chemical reactivities, which are at the heart of many of Nature's most amazing chemical transformations. My laboratory interrogates how metalloenzymes harness the reactivity of supernucleophiles and radical cofactors, while protecting themselves from potential damage. It is an incredibly exciting time to be studying metalloenzymes. Bioinformatics and genomic studies are identifying new putative metalloenzymes at a dizzying pace, with more than 100,000 unique sequences now associated with the Radical S-adenosylmethionine (SAM) enzyme family alone. Characterization of these enzymes is revealing unprecedented chemistry and new cofactor-binding structural motifs. Impressively, many of these Radical SAM (RS) enzymes are part of biosynthetic pathways that produce natural products with novel molecular scaffolds and promising pharmaceutical properties (including antibiotic, antiviral, and anti- tumor properties). My laboratory is employing our favorite technique of X-ray crystallography to probe sequence space within this family with the goal of understanding how RS enzymes harness radical-species to perform chemically challenging reactions. In the next five years, we will leverage recent success and continue to investigate the structure/function of cobalamin-dependent RS enzymes. This 7000-membered RS subgroup represents a new set of challenges and opportunities to understand how Nature tunes and controls both radical and supernucleophile reactivities. It is not only the RS enzyme family that has been in the spotlight recently; the glycyl radical enzyme (GRE) family is also receiving increased attention. In this latter case, the human microbiome project is providing new information as to the importance and abundance of GREs in the human gut and oral cavities. For example, the most abundant uncharacterized enzyme found in the gut is a GRE! In the next five years, we plan to investigate several newly discovered members of the GRE family that appear to be key players in human microbial communities. Our goal is to use our structural tools to interrogate the molecular basis for the radical-based chemistry that contributes to microbial metabolism, and potentially pathogenesis, in the human gut. A number of these GREs are found in common pathogens, like C. difficile, and are potential drug targets. Finally, it is a great period to be working on the ?great clusters of life,? which are responsible for the fixation of carbon (C-cluster/A-cluster), nitrogen (MoFe cluster) and hydrogen (H-cluster). My laboratory focuses on carbon fixation and the C- and A-clusters of carbon monoxide dehydrogenase/acetyl- CoA synthase. Recent advances have afforded recombinant systems that are allowing us to probe cluster assembly, reaction mechanism, and oxygen-sensitivity in a manner that was not possible previously. Oxygen- sensitivity is the Achilles heel of a complex metalloprotein and we plan to use our structural toolbox to investigate the molecular basis of C-cluster oxygen-sensitivity.